Devil May Cry 5 walkthrough/SM06

Location
On Mission 09: Genesis, after the second forced battle, go to the left side rather than going down on the right. There is one Blue Orb Fragment in the distance, locked by some Qliphoth roots. Summoning Nightmare will make him destroy the Blood Cloth inside this area which opens up the passage for V.

After obtaining the fragment, climb up the platform right in front of it, the Secret Mission area is up there.